Research has been largely directed toward the empirical induction of protection by live vaccines, together with studies on the protective capacity of humoral antibody against a virulent challenge. Inactivated ILT vaccines have also been used. However, due to the development of resistance and the difficulty in controlling the effect of virulent strains, new forms of vaccination and particularly new virulent strains of the virus are constantly being sought.
The interaction between ILT virus and the host immune system does not generally include infection of leukocytes or viraemia. Cell-mediated immunity is clearly the major mediator of ILT vaccinal protection, with additive effects likely for virus-neutralizing activity detectable in tracheal washings. Protection-inducing components of the ILT virus particle are presently being identified. Antibody studies indicate a substantial antigenic similarity between the major envelope glycoproteins of the wild-type and vaccine strains of ILT viruses.
Historically, the USA and Australia have shared an interest in the control of ILT virus infection since this arian respiratory disease syndrome was recognised in both countries in the mid-1920's. [Hanson, L. E. (1984) in: Diseases of Poultry, 8th ed. (M. S. Hofstad and others, eds.), pp 444-451. Iowa State University Press, Ames, Iowa]. The report by Hudson and Beaudette in 1933 of successful ILT vaccination via the vent [Hudson, C. B. and Beaudette, F. R. (1933) Cornell Veterinarian, 23: 63-65] was among the first examples of vaccinal control of an avian patbogen. However, some 40 years later in his review of ILT virus and the immune response, Hitchner [Hitchnet, S. B. (1975) American Journal of Veterinary Research, 36: 518-519] observed that nearly all subsequent ILT research had been directed towards the empirical aspects of developing partly-attenuated vaccines with assessment of their pathogenicity and the onset and duration of vaccinal protection.
Although it has been substantially controlled by the use of attenuated vaccines in the world's intensive poultry industries [Biggs, P. M. (1982) Arian Pathology,, 11: 281-300], [Hanson, L. E. (1984) In: Diseases of Poultry, 8th ed. (M. S. Hofstad and others, eds.), pp. 444-451. Iowa State University Press, Ames, Iowa], ILT infection and sporadic outbreaks of disease continue to cause difficulties, particularly in multi-age commercial sites.
The main disadvantage of known vaccines to the ILT virus is the high mortality rate caused by using pathogenic isolates. Previous ILT vaccines of low virulence have shown reduced ability to protect birds. The strain SA2 is one of the most commonly used vital strains in Australia at present. This strain has some side effects and can kill 2 to 3% of vaccinated broilers depending on the age of birds and method of husbandry and adminstration. Although the disease itself can kill 20% of a flock, the sporadic nature of the disease mitigates against the continued use of the SA2 vaccine with its residual virulence, thus enabling the disease to maintain its hold in the poultry population.